// 插件的写法

;(function() {
  let Compute = function() {};
  Compute.prototype = {
    plus(a, b) {
      return a + b
    },
    minus(a, b) {
      return a - b
    },
    mul(a, b) {
      return a * b
    },
    div(a, b) {
      return a / b
    },
  };
  window.Compute = Compute;
})()
let compute = new Compute();